Which measuring tool is used to determine workpiece diameter?

• A. Calipers
• B. Protractor
• C. Ruler
• D. Compass

Answer: (A)

Measuring the outside dimension of a round workpiece requires a tool that can grip around it and provide a precise reading across its widest span. Calipers fit that need perfectly. The external jaws clamp around the outside of the object to capture its diameter in one measurement, giving a direct reading in millimeters or inches. Digital calipers display the value clearly, while vernier calipers require reading the scale, but both give accurate, repeatable results for diameter measurements.

A protractor is for angles, not size. A ruler can measure length but lacks the precision and the proper spanning capability needed for an accurate diameter, especially on small or tightly toleranced pieces. A compass is mainly for drawing circles or transferring radii and isn’t reliable for measuring the actual diameter of a physical object.